The purpose of this study is to determine whether intra-coronary adenosine or sodium nitroprusside (SNP) delivered selectively via a thrombus aspiration catheter (or if unsuccessful via a coronary microcatheter) following thrombus aspiration in Primary Percutaneous Coronary Intervention (P-PCI) reduces microvascular obstruction (MVO) parameters and infarct size as measured with cardiac MRI, compared with standard treatment following thrombus aspiration in patients presenting with ST-elevation myocardial infarction (STEMI).